Polyester, and more particularly poly(butylene terephthalate) resin (PBT), is widely used to make meltblown nonwovens for applications such as filter media. PBT resins generally have good characteristics for meltblown processes; for example, thermal stability, fine fibers, and wettability. However, PBT meltblown webs have poor tensile strength, especially in the machine direction. In filter applications, the poor strength values require the web to be produced at prohibitive basis weights to maintain the integrity of the web. This is not practicable in the filter industry where the webs usually have to maintain a basis weight in the range of from 20 to 70 g/m2, while having an MD strength of at least about 250 g/cm. Accordingly, most PBT meltblown filters include a support scrim that is attached to one side of the web. The use of scrim backing increases the production cost of polyester filters.
While most PBT resins may be meltblown into fibers with relatively small diameters (approximately 3.5 μm), it is desired to produce filters with even finer fibers. Other problems with PBT meltblown webs include poor fiber binding as evidenced by excessive fly (loose fiber tufts), and poor web stiffness. Here again, improvements are desired because the presence of fly on the web surface makes the web difficult to handle, and low web stiffness negatively effects pleating, which is desired in filter applications.
The use of poly(ethylene terephthalate) resins (PET) for nonwoven filter media is not desirable because PET fibers exhibit poor heat stability. Meltblown PET webs are unable to withstand continuous high temperature operating conditions (i.e., 120° C.). The ability to withstand high temperature environments is essential in certain filter applications, e.g., oil filters. Furthermore, PET webs exhibit excessive shrinkage or other thermal deformation when subjected to high temperature transient conditions which may occur in filter fabrication. This may occur, for example, when melt bonding the web to an endpiece.
It has surprisingly been found that the properties of PBT meltblown webs can be vastly improved by introducing at least one additional aliphatic diol into a PBT-based resin. As described herein, at least one branched or cycloaliphatic diol is included in a polybutylene ester resin in addition to an aromatic dicarboxylic acid and butanediol.
References of interest include DE 934889 to Roelen et al. (1955), GB 847592 to Birch et al. (1957), Linear Polyesters (Farbwerke Hoechst A. G. 1964, U.S. Pat. No. 4,211,689 to Borman; U.S. Pat. No. 4,295,652 to Saito et al., U.S. Pat. No. 4,307,002 to Wagner et al.; U.S. Pat. No. 4,415,727 to Toga et al., U.S. Pat. No. 5,324,612 to Maeda et al., U.S. Pat. No. 5,916,677 to Chen et al., and U.S. Pat. No. 6,414,085 to Karas et al.